Playing it Cool with a Hot Rod Deluxe

Hi Jeff,
I’m sure you’ve heard it all before:
channel-switching problems, the reverb
drops in and out, unwanted distortion,
and volume fluctuations in a Fender Hot
Rod Deluxe. I bought mine used about
three years ago and enjoyed two glorious,
trouble-free years of music making. Then
all the aforementioned problems started
to happen. Plus the jewel light flickers
and randomly goes dim.

The internet forums—bet you love
those—point to a pair of 5-watt 470 Ω
resistors. From what I can tell, the general
consensus is to replace those with 330 Ω
resistors and keep them off of the board
with silicone.

Okay, I did all of that. These resistors
heat up like crazy, supposedly wreaking all
kinds of havoc, such as cold solder joints
and burned PC boards. I can attest to the
PC board. Now I understand that while
the 330 Ω resistors solve some problems,
they heat up even more. I know that
you’ve got the answer because you have a
column in the best magazine around!

Thanks,
Dennis

Hi Dennis,
Thanks for reading and for your kind words.
Per your assumption, yes, I’ve heard it
and seen it all before. Okay, probably not
all, but definitely this. The two resistors,
designated R85 and R86 in the “Blues”
series and R78 and R79 in the “Hot Rod”
series, are part of the low-voltage power
supply in these Fender amps.

While these Deluxe models are functionally
tube amps, there is some degree of
solid-state in the signal path. Not like modeling,
profiling, or sampling types of amps,
but in a much more basic way. All of the
clean, crunch, and overdriven tones in these
amps are generated by the good ol’ vacuum
tube, but there are a couple of places where
a solid-state device is used.

One of these locations is the effects
loop. Here, an op amp is used to buffer the
signal prior to the effects loop send and
another is used as a recovery preamp for
the effects loop return. Another area is the
reverb circuit: An op amp is used to generate
the send signal to the reverb tank, and
another handles the recovery of the reverb
signal from the tank. Also, switching ICs
(integrated circuits) control the amp’s FETs
(field-effect transistors) and relays.

The reason I mention all this is that
these solid-state devices are powered by the
amplifier’s low-voltage power supply. And
guess what? The low-voltage power supply
is derived through the two resistors in question.
So yes, issues like reverb dropouts and
volume drops or fluctuations are all potentially
caused by failure of the low-voltage
power supply. Let’s take a look at the
potential causes for this failure and what I’d
recommend doing to remedy it.

The two resistors in question. They can heat up
and cause solder joint failure, wreaking havoc
on the amp.

It’s true that the final low-voltage outputs,
+16VDC and -16VDC, come through these
resistors. In a typical scenario, when you use a
resistor to drop voltage—such as a plate resistor
in a tube circuit—the smaller the resistance
value, the less voltage drop across it.
Think of a decreasing resistor value as coming
closer to a piece of wire. A short piece of
wire will have virtually no voltage drop.

The original resistor values in your amp
were 470 Ω, and you mentioned a suggestion
to replace them with 330 Ω resistors.
Conventional wisdom dictates that with
less voltage drop across the resistors, they
should actually run at a cooler temperature
because any voltage that’s dropped across
the resistor is converted into heat.

But you say that the 330 Ω resistors actually
run hotter, and here’s why: These resistors do
not immediately or directly feed the low-voltage
circuit. The power supply uses two Zener
diodes on its output. These diodes “clamp” the
output voltage at a particular point—in this
case, 16 volts. According to the schematic, the
“input” voltage on the original 470 Ω resistors
is 48 volts. The output voltage is clamped at 16
volts. The voltage drop across each resistor is
32 volts. Using basic Ohm’s Law, voltage drop
squared divided by the resistance will give us
the power being dissipated across said resistor.

In this case, it’s 2.17 watts. If we do the
same calculation with the 330 Ω resistors,
we come up with 3.10 watts. Almost a full
watt more! So yes, you are correct, they will
run hotter. They are, of course, still within
their power rating of 5 watts, but the downside
is that the hotter they run, the more
potential damage they will do to the circuit
board ... especially because they are not in
an open-air environment.

I’d guess this forum repair suggestion references
the fact that the other amps in this family
(the Blues DeVille, Blues Deluxe, and Hot
Rod DeVille) all use 330 Ω resistors here.
However, the reason is that they’re all fed with
an initial DC voltage of approximately 33
volts, not 48, which allows them to run with
a cooler dissipation of less than 1 watt.

My suggestion would be to install the
correct 470 Ω resistors and keep them as far
away from the circuit board as possible. You
also need to check the integrity of the traces
on the board as well. These boards are constructed
with very thin, single-sided traces,
with no plated-through holes, and this makes
them very susceptible to damage, both from
overheated components, as well as simple
servicing. In instances like this, I’ve carefully
scraped the solder mask from the traces for a
good distance and added an extra-thick layer
of solder to reinforce the trace. It not only
helps to stabilize the connection, but also
helps to dissipate the heat at the leads.

Here’s hoping the only extra heat coming
off your Deluxe is from your playing!

Warning: All tube amplifiers contain lethal
voltages. The most dangerous voltages are
stored in electrolytic capacitors, even after the
amp has been unplugged from the wall. Before
you touch anything inside the amp chassis,
it’s imperative that these capacitors are discharged.
If you are unsure of this procedure,
consult your local amp tech.

Jeff Bober is one of the godfathers of the low-wattage amp revolution. He co-founded and was originally the principal designer for Budda Amplification, though he launched EAST Amplification (eastamplification.com) in 2010. You can catch his podcasts at ampsandaxescast.com or email him at pgampman@gmail.com.

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